GaN, AlN, and InN which are nitride semiconductors, materials composed of a mixed crystal thereof, and the like have a wide band gap, and thus have been used as a high-output electronic device, a short-wavelength light-emitting device, and the like. For example, GaN, which is a nitride semiconductor, has a band gap of 3.4 eV. The band gap is larger than a Si band gap of 1.1 eV and a GaAs band gap of 1.4 eV.
Examples of the high-output electronic device include a high electron mobility transistor (HEMT) as a field effect transistor (FET). An HEMT using a nitride semiconductor may be used for a high-output and high-efficiency amplifier, a large-power switching device, and the like. For example, in an HEMT in which AlGaN is used in an electron supply layer and GaN is used in an electron travel layer, piezoelectric polarization and the like occur in AlGaN due to distortion caused by a difference in lattice constant between AlGaN and GaN, and thus a high-concentration two-dimensional electron gas (2DEG) is generated. The HEMT having the above-described structure, that is, in the HEMT having a structure, in which the electron supply layer is formed from AlGaN and is formed on the GaN formed as the electron transit layer, is typically normally-on. However, in the above-described high-output electronic device, there is an application in which normally-off is demanded from the viewpoint of cost or fail-safe.
On the other hand, even in a semiconductor device having the structure other than the HEMT, a high-output electronic device may be manufactured by using a nitride semiconductor such as GaN. In the semiconductor device, a semiconductor device having a structure capable of realizing normally-off is also disclosed.
However, in the disclosed semiconductor device using GaN and the like, normally-off is possible, but a withstand voltage may not be sufficient, and thus the semiconductor device may not be suitable as a high-power switching device in some cases.
The following are reference documents:    [Document 1] Japanese Laid-open Patent Publication No. 2002-359256,    [Document 2] Japanese Laid-open Patent Publication No. 2008-205414 and    [Document 3] Japanese Laid-open Patent Publication No. 2013-55148.